The invention relates to a method for growing xcex1-SiC bulk single crystals, wherein the bulk single crystal is formed from an SiC gas phase by the deposition of SiC on an SiC seed crystal. In this context, the term SiC gas phase is understood as meaning a gas phase comprising the components Si, SiC, Si2C and SiC2. The term xcex1-SiC bulk single crystals includes crystals of rhombohedral and hexagonal structure.
Silicon carbide (SiC) is the typical example of a substance which exhibits the appearance of polytypism (single-dimensional polymorphism). The literature has disclosed over 200 polytypic modifications, which are referred to here as polytypes. The polytypes have different physical properties, such as energy gap, electron mobility, and optical properties.
The most widely known polytypes are those which bear the designations 4H, 6H, 3C and 15R. In particular, the three polytypes 4H, 6H and 15R clearly have the same enthalpy of formation and therefore the same thermodynamic stability. Of these three polytypes, 4H and 6H are more common than the polytype 15R. It can therefore be assumed that the polytype 15R has a slightly lower thermodynamic stability than the other two polytypes.
Various methods for producing a 4H or 6H SiC bulk single crystal have become known. For example, U.S. Pat. No. 4,866,005, reissued as Re34861 (corresponding to European patent application EP 0 712 150 A1) and international PCT publication WO 97/27350 describe the production of a 6H SiC bulk single crystal. There, a 6H SiC seed crystal is used with a growth surface whose normal is tilted through 3xc2x0 toward the [0001] crystal direction.
For some electronic applications, the 15R polytype has advantageous properties which make it of interest in particular for the fabrication of, for example, MOS transistors.
Chien, Nutt, Yoo, Kimoto, and Matsunami, describe, in Journal of Mater. Res. 9 (1994) 940 that 3C layers grow on 15R substrates without double position boundaries (DPBs). However, the reproducible production of SiC bulk single crystals is very difficult. Hitherto, 15R inclusions in substrate wafers have been purely random during production and were impossible to generate reproducibly.
In practice, it has not hitherto been possible to produce 15R crystals by means of epitaxy. This is so because in the case of epitaxial growth on a substrate with off-axis orientation, the epitaxial layers generally adopt the same modification as that which is predetermined by the substrate. By contrast, in the case of epitaxial growth on a substrate without off-axis orientation, it is predominantly the cubic modification, i.e. the 3C modification, which grows. Irrespective of this, only relatively thin layers can be produced with acceptable outlay using epitaxy.
It is accordingly an object of the invention to provide a method of growing an SiC bulk single crystal, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which can be used to grow SiC bulk single crystals of the type 15R reproducibly and without restriction to the seed crystal, so that the SiC bulk single crystals are suitable as a substrate for a semiconductor component (under certain circumstances with a following epitaxial layer).
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of growing an xcex1-SiC bulk single crystal, which comprises:
depositing SiC from gas phase on a seed crystal and growing a bulk single crystal under uniaxial tensile stress enclosing a predetermined angle with a [0001] axis of the bulk single crystal, and thereby forming a rhombohedral crystal.
The invention is based on the idea of exposing the growing crystal to a uniaxial tensile stress which has a component in the  less than 11{overscore (2)}0 greater than  direction. In this context,  less than 11{overscore (2)}0 greater than  denotes a vector in the reference system of the crystal which points in the [11{overscore (2)}0] direction, the numbers in the pointed or square brackets being the indices. The use of the pointed brackets means that all symmetrically equivalent vectors in this crystal system are intended to be indicated.
The method according to the invention for growing xcex1-SiC bulk single crystals, wherein the bulk single crystal is formed from an SiC gas phase by deposition of SiC on an SiC seed crystal is characterized in that the deposition takes place under a uniaxial tensile stress which includes a predetermined angle with the [0001] axis of the bulk single crystal.
In a first preferred embodiment, the uniaxial tensile stress is generated by a temperature field, the axial gradient of which includes the predetermined angle with the [0001] axis of the bulk single crystal. Furthermore, in the axial direction the temperature field has a non-vanishing second position derivative of the temperature. It is therefore the case that d2T/dx2xe2x89xa00, where T denotes the temperature and x denotes the axial position coordinate. In this context, the term axial means in the direction of growth, while radial accordingly represents an orientation which is perpendicular to the direction of growth.
In accordance with a further preferred embodiment, the uniaxial tensile stress is generated by a mass flow of SIC which, by the expedient of a diaphragm system, is directed onto the bulk single crystal at the predetermined angle with respect to the [0001] axis of the bulk single crystal.
In accordance with again a further preferred embodiment, the uniaxial tensile stress is generated by orientation of the seed crystal, so that the normal to a growth surface of the seed crystal includes the predetermined angle with the [0001] axis of the bulk single crystal. In this case too, the temperature field has a non-vanishing second position derivative of the temperature in the axial direction. The relationship d2T/dx2xe2x89xa00 applies once again. The growth surface of the seed crystal is preferably inclined in a  less than 11{overscore (2)}0 greater than  direction by the predetermined angle.
The predetermined angle is preferably between 2xc2x0 and 10xc2x0.
In accordance with yet again a further preferred embodiment, the uniaxial tensile stress is generated by orientation of the seed crystal, so that the normal to a growth surface of the seed crystal includes the predetermined angle with the [0001] axis of the bulk single crystal. In this case, in addition to this special orientation of the seed crystal, an inhomogeneous radial temperature profile, i.e. a radial temperature gradient, is established, so that curved isotherms result. The curved isotherms run substantially in the radial direction. To generate the uniaxial tensile stress, it is particularly expedient if the isotherms have a radius of curvature which is at most 4 times the diameter of the bulk single crystal.
In a further preferred embodiment, the uniaxial tensile stress is generated by fitting the bulk single crystal into a tube during growth. The tube then exerts a non-uniform pressure on the growing crystal.
The seed crystal is preferably oriented in such a manner that the growth of SiC takes place on that side of the seed crystal on which the Si atoms are situated.
One advantage of the method according to the invention consists in the fact that xcex1-SiC bulk single crystals, in particular of type 15R, can be grown reproducibly and with a high yield with relatively little outlay, while amazingly the polytype of the seed used is of little or no importance.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for growing an xcex1-SiC bulk single crystal, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.